N.C. State University leads research into kudzu bug host preferences

For an exotic invasive insect pest, kudzu bug is fairly easy to control. Spray a pyrethroid, and it’s gone.

The problem is that the pyrethroid also takes with it many beneficial insects that usually keep other soybean pests low in numbers. So although the field may be free of kudzu bugs, it could later be overrun with soybean looper and caterpillar pests that are just as destructive as kudzu bug. So the grower has to keep spraying.

N.C. State University Extension Specialist Dominic Reisig wants to find a way to keep growers with kudzu bug problems out of the “spray continuum.” So he and his colleagues from South Carolina and Georgia will use a $168,644 U.S. Department of Agriculture Southern Regional IPM grant to find out why kudzu bugs leave their home in kudzu patches to move to soybean fields.

Kudzu bug has been in the U.S. only since 2009, where hundreds of them covered the outside of houses in Atlanta, Ga. From there it has spread throughout the South, covering the entirety of the Carolinas and Georgia, and parts of Virginia, Tennessee, Alabama, Florida, Mississippi and Louisiana.

Although it feeds on the voraciously invasive kudzu weed, the pest is a nuisance in residential areas and a disaster in a soybean field. Kudzu bugs reduce soybean yields on average about 20 percent in a field that hasn’t been sprayed, and they reproduce so rapidly that a field can be overrun with them in just a few days. So many farmers spray an insecticide when they see kudzu bugs start to multiply, only to find that the rest of the growing season is a continuing battle with other pests.

Kudzu bug got its name for its affinity for kudzu, in which the bug lives after coming out of its overwintering slumber. In May, kudzu bug migrates to soybean fields.

“Kudzu bugs overwinter behind tree bark,” Reisig says. “They particularly like the loose bark of pine trees. We’ve also found them below broadleaf litter. From there they move to kudzu, so we think kudzu is an important bridge host.”

Reisig and his colleagues are especially interested in why kudzu bug leaves kudzu in the first place, along with the distance that the pest can fly between a kudzu patch and the nearest soybean field.

“We want to know what kind of risk a grower is in if the soybean field is near a kudzu patch,” Reisig says.

To measure how far kudzu bugs can fly, researchers will attach adult kudzu bugs to a flight mill. Similar to tiny trapeze artists, the bugs will spin around a pole, suspended from a string attached to a rotating arm. The device measures the insect’s speed and distance.

The test will tell researchers how far the insect can fly to find a soybean field.

“Is it important that you’re near a kudzu patch or not?” says Reisig. “If they can fly 50 miles, then it doesn’t matter where kudzu is in relation to your field. If they don’t fly very far away from kudzu, then maybe staying local is more important to them than long distance movement.”

The big mystery is why kudzu bugs leave kudzu in the first place. Does something about kudzu motivate them to leave it? Is the insect motivated by temperature or humidity? Or does it just get too crowded in the kudzu patch, forcing groups of the pest to find a new home?

To find out more, researchers will explore how each potential host affects the fitness of the insect.

To test the effect of the host on the insect’s longevity, researchers will rear maturing kudzu bugs in two settings: one on kudzu and one on soybeans. The scientists will observe how long an insect lives on each host and then compare the two.

Next, researchers will pair male and female kudzu bugs in a cage over potted soybean and kudzu seedling and observe where the female lays her eggs. They will then assess the fitness of the hatched larvae by repeating the longevity and flight test for the new generation. They will also observe whether the second generation makes similar egg-laying choices as the parents.

In late March, armed with air blowguns filled with a fluorescent powder, Reisig and colleagues in South Carolina and Georgia will survey kudzu patches, dusting the powder on any kudzu bugs they find. Each patch of kudzu will be dusted with a different colored powder to identify the kudzu patch. Temperature and moisture sensors in the patches will measure hourly temperature and humidity to find if either affects kudzu bug’s movement out of the kudzu patch.

The researchers will then set up traps in selected soybean fields both near and far away from kudzu patches, checking them in April and May for fluorescent and non-fluorescent bugs.

“It’ll be interesting to find out what proportion of marked and unmarked individuals are in our traps to see where the bugs are coming from,” Reisig says.

A separate experiment will cage nymphs from kudzu-preferring females and deliberately overcrowd them to see if insect density affects their quality of life.

A previous project funded by the United Soybean Board tested various planting dates by month, from April to June, to observe whether kudzu bug preferred soybeans planted in a certain month. The bug preferred the crop planted in April over the other two plantings.

The problem is that most growers don’t plant soybeans in April. So the question is whether kudzu bug populates kudzu in April, just waiting for soybeans in May or June, or if the pest needs something from the kudzu vines before it moves to soybeans.

Reisig wants to conduct a more realistic test of planting dates, varying them by week rather than month, and concentrating dates in May and June. He hopes to find out if the pest still has a preference for the crop based on planting date. That information could inform growers about the best time to scout for kudzu bug. Depending on the outcome of the experiment, some growers may opt to alter their planting date.

“The looming question for this project is how important kudzu is as a host,” Reisig says. “If kudzu is important, the pest will be restricted to the states that have kudzu. If it’s not, kudzu bug will eventually spread to the Midwest. So this project will have wide implications once we figure out the role of kudzu in the biology of the kudzu bug.”